To the Editor: Although Dictyostelium spp. are used for studying
signal transduction, cytoskeletal functions, endocytosis, and molecular
pathogenesis of infectious and other diseases (1), human or animal
infections caused by this organism have not been reported. We report a
case of keratitis caused by Dictyostelium polycephalum in an
immunocompetent person.

A 35-year-old man sought treatment for redness, pain, and watering
in the left eye of 11 days' duration. He had no history of ocular
injury or surgery. At the time of his medical visit, he was using
ophthalmic solutions of 5% natamycin sulfate, 0.5% moxifloxacin
hydrochloride, and 0.3% gentamicin sulfate, each instilled every hour,
and 1% atropine sulfate, 3x/d.

The vision in his right eye and results of a clinical examination
were within normal limits. His left eye visual acuity was expressed as
the ability to count fingers at 1 m. The eyelids were edematous and the
conjunctivae were congested. The cornea showed a large central
epithelial defect with underlying stromal infiltrate and Descemet folds.
The surrounding cornea had a mild cellular reaction. The anterior
chamber was deep, and the pupil was round, regular, and dilated. Iris
and lens details could not be distinguished because of corneal haze. We
obtained corneal scrapings, and the material was subjected to a detailed
microbiologic analysis (2).

Microscopic examination showed double-walled spherical cysts in
potassium hydroxide with calcofluor white stain, Gram stain (Figure,
panels A, B), and Giemsa stain. On the basis of this finding, a
presumptive diagnosis of Acanthamoeba keratitis was made. The patient
was advised to use 0.02% polyhexamethylene biguanide and 0.02%
chlorhexidine eye drops every half hour and 1% atropine eye drops 3x/d
and was asked to return for a follow-up visit the next day. However, the
patient did not return and could not be located. After 48 hours' of
incubation, a nonnutrient agar plate showed growth of double-walled,
spherical cysts [approximately or equal to] 6-7 urn in diameter that had
different morphologic features than those of Acanthamoeba spp. cysts.

To identify the organism, we extracted DNA from the growth on
nonnutrient agar and subjected it to PCR specific for Acanthamoeba spp.
(3); results were negative. The extracted DNA was then subjected to 18S
rDNA PCR for free-living amebas as described by Tsvetkova et al. (4). A
PCR product [approximately or equal to] 800 bp was obtained and
subjected to bidirectional sequencing with fluorescent-labeled dideoxy
nucleotide terminators by using ABI 3130 XI automated sequencer in
accordance with the manufacturer's instructions (PE Applied
Biosystems, Foster City, CA, USA).

The Mega BLAST search program (www.ncbi.nlm.nih.gov/blast/
megablast.shtml) of GenBank identified the sequence as D. polycephalum
(99% similarity with AM168056). We deposited the sequence of our isolate
in GenBank (accession no. GU562439). The organism showed cytotoxicity
after in vitro inoculation of a rabbit corneal epithelial cell line.

The patient sought treatment 4 months after his initial visit. The
left eye visual acuity was now expressed as the ability to see hand
movements near the face. Slit-lamp examination showed lid edema and
conjunctival congestion. The cornea showed a ring-shaped infiltrate,
central thinning, surrounding corneal edema, and pigments on the
endothelium (Figure, panel C); these findings were identical to the
clinical picture of Acanthamoeba keratitis. Repeat corneal scrapings
showed organisms of same morphologic features seen on the first visit by
microscopy and culture. Organisms were reidentified as D. polycephalum
by sequencing.

[FIGURE OMITTED]

Because we were not aware of any drug treatment recommendations for
infection by this organism, and the disease was advanced, surgical
treatment was advised. Deep anterior lamellar keratoplasty was performed
after 2 days. Histopathologic examination of the corneal button showed
spherical cysts in mid stroma and inflammatory infiltrates (Figure,
panel D). At the last follow-up (3 months after surgery), the corneal
graft was clear with no evidence of infection.

Members of the genus Dictyostelium (social amebas or cellular slime
molds) are divided into 4 high-level taxa with several species on the
basis of DNA phylogeny (5). The life cycle of Dictyostelium spp.
consists of an ameboid vegetative phase, a cyst phase, and a plantlike
fruiting phase (6). D. polycephalum is ancestral and shows different
characteristics than other species of Dictyostelium (5,7,8). In culture,
it grows at a temperature of 34[degrees]C-35[degrees]C, which is higher
than that for other species of Dictyostelium (8). Most myxamoebae
aggregate to form sporocarps; however, some may round up in individual
cells to form microcysts (8). The D. polycephalum isolated from our
patient grew at 36[degrees]C on nonnutrient agar with an Escherichia
coli overlay. The myxamoebae were seen after 24 hours, and the amebae
had transformed into microcysts after 48 hours of incubation. However,
on further incubation for 3 weeks at 36[degrees]C, no sporocarp formed.

Although we could identify the microorganism, the source of
infection is unknown. Because the patient was a manual laborer, he could
have become infected with the organism from contaminated water or soil.
The clinical picture for keratitis caused by D. polycephalum was
indistinguishable from that caused by Acanthamoeba spp. However, careful
attention to cyst morphology in clinical samples and culture enabled us
to identify this organism.

This study was supported by the Hyderabad Eye Research Foundation,
Hyderabad, India.